[FEATURE] API call to make geometries valid (port of ST_MakeValid)

This introduces QgsGeometry::makeValid() which will try to make a valid
geometry out of invalid one. This is more complicated method than just
doing a buffer with zero width, but it should not loose any vertices.

Finally we should have a reliable way in QGIS to fix bad geometries!

Ported the C code from lwgeom library to QGIS.

python/core/geometry/qgsgeometry.sip

@@ -747,6 +747,19 @@ class QgsGeometry
      */
     int avoidIntersections( const QList<QgsVectorLayer*>& avoidIntersectionsLayers );
 
+    /**
+     * Attempts to make an invalid geometry valid without losing vertices.
+     *
+     * @note Ported from PostGIS ST_MakeValid() and it should return equivalent results.
+     * Already-valid geometries are returned without further intervention.
+     * In case of full or partial dimensional collapses, the output geometry may be a collection
+     * of lower-to-equal dimension geometries or a geometry of lower dimension.
+     * Single polygons may become multi-geometries in case of self-intersections.
+     * @return new valid QgsGeometry or null geometry on error
+     * @note added in QGIS 3.0
+     */
+    QgsGeometry makeValid();
+
     class Error
     {
       public:

src/core/CMakeLists.txt

@@ -361,6 +361,7 @@ SET(QGIS_CORE_SRCS
   geometry/qgsgeometrycollection.cpp
   geometry/qgsgeometryeditutils.cpp
   geometry/qgsgeometryfactory.cpp
+  geometry/qgsgeometrymakevalid.cpp
   geometry/qgsgeometryutils.cpp
   geometry/qgsgeos.cpp
   geometry/qgsinternalgeometryengine.cpp

src/core/geometry/qgsgeometry.cpp

@@ -22,6 +22,7 @@ email                : morb at ozemail dot com dot au
 #include "qgsgeometry.h"
 #include "qgsgeometryeditutils.h"
 #include "qgsgeometryfactory.h"
+#include "qgsgeometrymakevalid.h"
 #include "qgsgeometryutils.h"
 #include "qgsinternalgeometryengine.h"
 #include "qgsgeos.h"
@@ -1887,6 +1888,21 @@ int QgsGeometry::avoidIntersections( const QList<QgsVectorLayer*>& avoidIntersec
   return 0;
 }
 
+
+QgsGeometry QgsGeometry::makeValid()
+{
+  if ( !d->geometry )
+    return QgsGeometry();
+
+  QString errorMsg;
+  QgsAbstractGeometry* g = _qgis_lwgeom_make_valid( *d->geometry, errorMsg );
+  if ( !g )
+    return QgsGeometry();
+
+  return QgsGeometry( g );
+}
+
+
 void QgsGeometry::validateGeometry( QList<Error> &errors )
 {
   QgsGeometryValidator::validateGeometry( this, errors );

src/core/geometry/qgsgeometry.h

@@ -802,6 +802,19 @@ class CORE_EXPORT QgsGeometry
     int avoidIntersections( const QList<QgsVectorLayer*>& avoidIntersectionsLayers,
                             const QHash<QgsVectorLayer*, QSet<QgsFeatureId> >& ignoreFeatures = ( QHash<QgsVectorLayer*, QSet<QgsFeatureId> >() ) );
 
+    /**
+     * Attempts to make an invalid geometry valid without losing vertices.
+     *
+     * @note Ported from PostGIS ST_MakeValid() and it should return equivalent results.
+     * Already-valid geometries are returned without further intervention.
+     * In case of full or partial dimensional collapses, the output geometry may be a collection
+     * of lower-to-equal dimension geometries or a geometry of lower dimension.
+     * Single polygons may become multi-geometries in case of self-intersections.
+     * @return new valid QgsGeometry or null geometry on error
+     * @note added in QGIS 3.0
+     */
+    QgsGeometry makeValid();
+
     /** \ingroup core
      */
     class Error